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Bear Muscles Bear the Winter

It takes the strength of a bear to stay fit without exercise. Although humans quickly lose power in their muscles if they don't use them, black bears retain almost all their strength during their long hibernation, according to a new study. The phenomenon may someday help researchers prevent muscle degeneration in humans during bed rest, limb healing, or space flight.

In winter, bears are among the most inert mammals known; they don't even urinate during their 5- to 7-month-long hibernation. In a previous study, a team that included physiological ecologist Henry Harlow of the University of Wyoming, Laramie, had taken biopsies from bears early and late during hibernation. They found that muscles retained their cell number and size, and many lost no protein over the winter.

Now, Harlow's group has shown that bear muscles also keep their strength. During the summer, they hooked up bears with transmitters, using their signals to find the bears' well-hidden dens during the winter. After sedating an already sleeping bear, the researchers measured how much one of its leg muscles contracted when they stimulated a nerve. In the 22 February issue of Nature, they report that strength waned by only 23% between the beginning of hibernation in late autumn and its end 130 days later. Humans inactive for such a long time would become much weaker; extrapolating from shorter term studies of bed rest and space flight, the team estimates they would lose 90% of their muscle strength.

What do bears have that we don't? The authors don't know, but they suggest several possibilities. The bears may digest parts of their own underused digestive tract in the winter, or they may recycle urea, a product of protein breakdown, back into muscle proteins. The team also speculates that bears could keep their muscles toned by contracting them or by shivering. "It's anybody's guess how they are doing it," says Reggie Edgerton, a physiologist at the University of California, Los Angeles, who has studied muscle atrophy in humans during space flight.

Perhaps researchers could learn from the bears, Edgerton says: "If we found out the mechanisms, we could reduce the amount of atrophy in long-term bed rest and other conditions." One of those conditions: a future mission to Mars, which could take as long as 5 years and would be a serious assault on astronauts' muscle power, Edgerton says.